=129. Pavy’s Process.=—The well-known solubility of cuprous oxid in
ammonia led Pavy to adopt a copper reagent containing ammonia in the
volumetric determination of reducing sugars.[95] In Pavy’s process an
alkaline copper solution is employed made up in the usual way, to which
a sufficient quantity of ammonia is added to hold in solution all the
copper when precipitated as cuprous oxid. The solution used by Pavy has
the following composition: One liter contains
Crystallized copper sulfate 34.65 grams
Potassium-sodium tartrate 173.00 ”
Caustic potash 160.00 ”
For use 120 cubic centimeters of the above reagent are mixed with 300
of ammonia of specific gravity 0.88, and the volume completed to one
liter with distilled water. Twenty cubic centimeters of this reagent
are equivalent to ten milligrams of dextrose or invert sugar when added
in a one per cent solution.
In the use of ammoniacal copper solution, care must be taken that
all the liquids employed be entirely free of oxygen and that the
contents of the flask in which the reduction takes place be in some way
excluded from contact with the air. Pavy secured this by conducting
the reduction in a flask closed with a stopper carrying two holes;
one of these served for the introduction of the burette carrying the
sugar solution and the other carried a tube dipping into a water seal
by means of a slit rubber tube, which would permit of the exit of the
vapors of steam and ammonia, but prevent the regurgitation of the water
into the flask.
The complete decoloration of the copper solution marks the end of the
reaction. The usual precautions in regard to the length of the time of
boiling must be observed.
It is easy to see that in the Pavy process the quantity of ammonia in
the solution is rapidly diminished during the boiling and this has led
to the suggestion of other methods to exclude the air. Among these
have been recommended the introduction of a current of hydrogen or
carbon dioxid. One of the best methods of procedure is that proposed
by Allen, who recommends covering the copper solution by a layer of
paraffin oil (kerosene).[96]
=130. Process Of Peska.=—Peska has also independently made use of
Allen’s method of covering the solutions with a layer of paraffin oil
and finds it reliable.[97] The copper reagent employed by him has the
following composition:
Crystallized copper sulfate 6.927 grams
Ammonia, twenty-five per cent strength 160.00 cc.
The copper sulfate is dissolved in water, the ammonia added, and
the volume completed to half a liter with distilled water. A second
solution containing half a liter is made by dissolving 34.5 grams
of potassium-sodium tartrate and ten grams of sodium hydroxid and
completing the cool solution to half a liter with distilled water.
In all cases the water used in making up the above solutions must be
freshly boiled to exclude the air.
For the titration, fifty cubic centimeters of each of the above
solutions are taken, mixed and covered with a layer of paraffin oil
half a centimeter in depth. The reduction is not accomplished at a
boiling temperature, but at from 80° to 85°. The manipulation is
conducted as follows:
The mixed solutions are placed in a beaker, covered with oil, and
heated to 80°. The temperature is measured by a thermometer which also
serves as a stirring rod. The sugar solution is run down the sides
of the beaker from a burette of such a shape as to be protected from
the heat. After each addition of the sugar solution the mixture is
carefully stirred, keeping the temperature at from 80° to 85°. The
first titration is made to determine approximately the quantity of
sugar solution necessary to decolorize the copper. This done, the
actual titration is accomplished by adding at once the total amount
of sugar solution necessary to decolorize, less about one cubic
centimeter. Any sugar solution adhering to the side of the beaker
is washed down by distilled water, the contents of the beaker well
stirred, and the temperature kept at 85° for two minutes. The rest of
the sugar solution is then added in quantities of one-tenth of a cubic
centimeter until the decoloration is completed. The total time of the
final titration should not exceed five minutes. The sugar solution
should be as nearly as possible of one per cent strength. If a lower
degree of strength be employed a larger quantity of the sugar is
necessary to reduce a given quantity of copper.
In the case of dextrose, when a one per cent solution is used, eight
and two-tenths cubic centimeters, corresponding to 80.2 milligrams of
dextrose, are required to reduce 100 cubic centimeters of the mixed
reagent. On the other hand, when the sugar solution is diluted to
one-tenth of a per cent strength 82.1 milligrams are required.
With invert sugar slightly larger quantities are necessary, the
reducing power being as 94.9 to 100 as compared with dextrose. With
a one per cent strength of invert sugar it is found that eighty-four
milligrams are required to reduce 100 cubic centimeters of the mixed
reagent and when the strength of the invert sugar is reduced to
one-tenth per cent 87.03 milligrams are required.
=131. Method Of Allein and Gaud.=—Allein and Gaud have proposed a
further modification of the ammonia process which consists essentially
in the suppression of rochelle salt and fixed caustic alkali and the
entire substitution therefor of ammonia. Ammonia acts with much less
vigor upon sugars than the caustic alkalies, and it is therefore
claimed that the decomposition of the sugar due to the alkali is
reduced to a minimum when ammonia is employed.[98] The copper solution
is made as follows:
Dissolve 8.7916 grams of electrolytic copper in ninety-three grams
of concentrated sulfuric acid diluted with an equal volume of water.
Complete the resulting solution to one liter with concentrated ammonia.
Ten cubic centimeters of this solution are equal to fifty grams of
dextrose.
It is recommended that the reduction be accomplished in an atmosphere
of hydrogen, but it is apparent that the use of kerosene is permissible
in this case, and on account of its greater simplicity it is to be
recommended as the best means of excluding the oxygen. The reduction is
accomplished at a temperature of about 80°.
It is also proposed to reoxidize the copper by substituting a current
of air for the hydrogen at the end of the reaction, and thus use the
same copper a number of times. The danger of loss of ammonia, and the
difficulty of determining when the oxidation is complete, render this
regeneration of the reagent undesirable.
=132. Method of Gerrard.=—The method of Gerrard does not depend upon
the use of ammonia, but the principle involved is the same, _viz._, the
holding of the separated cuprous oxid in solution and the determination
of the end of the reaction by the disappearance of the blue color.
As first proposed by Gerrard, the copper sulfate solution is made of
double the strength usually employed and to each 100 cubic centimeters
thereof, before use, three and three-tenths grams of potassium cyanid
are added. This is sufficient to hold the precipitated cuprous oxid in
solution.[99]
The original method of Gerrard is found difficult of execution and the
author, in conjunction with Allen, has lately modified it and reduced
it to a practical working basis.[100]
In the new method the ordinary fehling solution is employed and it
is prepared for use in the following way: Ten cubic centimeters of
the fehling solution, or half that quantity of each of the component
parts kept in separate bottles, are placed in a porcelain dish with
forty cubic centimeters of water and brought to the boiling-point. To
the boiling liquid is added, from a pipette, a five per cent solution
of potassium cyanid until the blue color just disappears, or only a
very faint tint of blue remains, avoiding any excess of the cyanid. A
second portion of the fehling solution equal to that first employed
is added, and to the boiling mixture the solution of sugar is added,
from a burette, until the blue color disappears. The contents of the
dish should be kept boiling during the addition of the sugar solution.
The volume used will contain fifty milligrams of dextrose. The sugar
solution should be of such a strength as to contain no more than half a
per cent of reducing sugar.
The principle of the preparation of the solution may be stated as
follows: If to a solution of copper sulfate, potassium be added until
the blue color disappears, a double cyanid of copper and potassium
cyanid is formed according to the following reaction: CuSO₄ + 4KCN =
Cu(CN)₂.2KCN + K₂SO₄. This double cyanid is a salt of considerable
stability. It is not decomposed by alkalies, hydrogen or ammonium,
sulfid. With mineral acids it gives a whitish, curdy precipitate. With
fehling solution the same double cyanid is formed as that described
above. If, however, fehling solution be present in excess of the amount
necessary to form the double cyanid of copper, this excess can be used
in the oxidation of reducing sugar and the colorless condition of the
solution will be restored as soon as the excess of the fehling is
destroyed. The double cyanid holds in solution the cuprous oxid formed
and thus complete decoloration is secured.
=133. Sidersky’s Modification of Soldaini’s Process.=—In all cases
where the sugar solutions are not too highly colored, Sidersky finds
that the method of reduction in a large test tube, as practiced by
Violette, is applicable with the copper carbonate solution.[101] For
more exact work it is preferred to determine the quantity of copper
reduced by an indirect volumetric method. The sugar solution, properly
clarified and the lead removed if subacetates have been used, is made
of such a volume as to contain less than one per cent of reducing
sugars. In a flask or large test tube are placed 100 cubic centimeters
of the copper solution, which is boiled for a short time and the sugar
solution added, little by little, from a pipette, at such a rate as
not to stop the ebullition. The boiling is continued for five minutes
after the last addition of the sugar. The vessel is taken from the
flame and 100 cubic centimeters of cold water added, the whole brought
on an asbestos felt and the cuprous oxid washed with hot water until
the alkaline reaction has disappeared. The residual cuprous oxid
is dissolved in a measured quantity of set sulfuric acid, semi- or
fifth-normal, a few particles of potassium chlorate added, and the
mixture boiled to convert any cuprous into cupric sulfate. The reaction
is represented by the following formula:
3Cu₂O + 6H₂SO₄ + KClO₃ = 6CuSO₄ + KCl + 6H₂O.
The residual sulfuric acid is titrated with a set alkali in excess,
ammonia being preferred.
The solution of ammonia is made by diluting 200 cubic centimeters of
commercial aqua ammonia with 800 of water. Its strength is determined
by adding a little copper sulfate solution as indicator and then
the set solution of sulfuric acid until the blue color disappears.
The copper sulfate secured from the cuprous sulfate as described
above is cooled, and a quantity of the ammonia, equal to twenty-five
cubic centimeters of the set sulfuric acid, added. The excess of the
ammonia is then determined by titration with the sulfuric acid, the
disappearance of the blue color being the indication of the end of the
reaction. The number of cubic centimeters of the set sulfuric acid
required to saturate the ammonia represents the equivalent of cuprous
oxid originally present. One cubic centimeter of normal sulfuric acid
is equivalent to 0.0317 gram of metallic copper.
To determine the weight of invert sugar oxidized, multiply the weight
of copper, calculated as above described, by the factor 0.3546.[102]
For a general application of this method of analysis the relative
quantities of copper reduced by different quantities of sugar must be
taken into consideration.
While, as has already been stated, the copper carbonate process has
heretofore been applied chiefly to the detection of invert sugar, it
has merits which justify the expectation that it may some time supplant
the fehling liquor both for volumetric and gravimetric work. Large
volumes of the reagent can be prepared at once and without danger of
subsequent change. The action of the reagent on the hexobioses and
trioses is far less vigorous than that of the alkaline copper tartrate,
and the end reactions for volumetric work are, at least, as easily
determined in the one case as the other.
=134. Method Depending on Titration of Excess of Copper.=—Instead of
measuring the quantity of copper reduced, either by its disappearance
or by reducing the cuprous oxid to a metallic state, Politis has
proposed a method of analysis depending on the titration of the
residual copper.[103] The reagents employed are:
(1) A copper solution containing 24.95 grams of crystallized copper
sulfate, 140 grams of sodium and potassium tartrate, and twenty-five
grams of sodium hydroxid in one liter:
(2) A solution of sodium thiosulfate containing 24.8 grams of the salt
in one liter:
(3) A solution of potassium iodid containing 12.7 grams of iodin in one
liter.